Reijo Visuri

Erythemally weighted radiometers in solar UV monitoring: Results from the WMO/STUK intercomparison

The first international intercomparison of erythemally weighted (EW) broadband radiometers was arranged in 1995 to improve the accuracy and comparability of the measurements carried out by solar UV monitoring networks. The intercomparison was arranged at the Radiation and Nuclear Safety Authority in Helsinki, Finland, in cooperation with the University of Innsbruck and with support from the World Meteorological Organization. Altogether 20 EW meters of six different types from 16 countries were (1) tested in the laboratory by measuring the spectral and angular responsivities and (2) calibrated in solar radiation against two reference spectroradiometers. Calibration factors (CFs) for the EW meters were determined by using simultaneously measured EW solar UV spectra as a calibration reference. The CFs averaged over solar elevations higher than 35° varied from 0.87 to 1.75, with the estimated uncertainty being ±10%. As a result of this intercomparison, for the first time the calibrations of more than 100 EW radiometers around the world are possible to trace to the same origin. The present experience indicates that the accuracy of temperature‐controlled EW radiometers is not significantly lower than the accuracy of spectroradiometers provided that strict quality assurance/quality control procedures are followed.

Evaluation of a single-monochromator diode array spectroradiometer for sunbed UV-radiation measurements.

The suitability of a new technology single‐monochromator diode array spectroradiometer for UV‐radiation safety measurements, in particular for sunbed measurements, was evaluated. The linearity, cosine response, temperature response, wavelength scale, stray‐light and slit function of the spectroradiometer were determined and their effects on the measurement accuracy evaluated. The main error sources were stray‐light and nonideal cosine response, for which correction methods are presented. Without correction, the stray‐light may reduce the accuracy of the measurement excessively, particularly in the UV‐B range. The expanded uncertainty of the corrected UV measurements is estimated to be 14%, which is confirmed with the comparative measurements carried out with a well‐characterized double‐monochromator spectroradiometer. The measurement accuracy is sufficient for sunbed measurements, provided that all corrections described above have been done and the user of the instrument has a good understanding of the instruments operating principles and potential error sources. If these requirements are met, the tested spectroradiometer improves and facilitates market surveillance field measurements of sunbeds.

EFFECTS OF ARCTIC OZONE DEPLETION AND SNOW ON UV EXPOSURE IN FINLAND

The increase in the UV exposure of the Finnish population associated with the combined effects of ozone depletion and snow reflection was studied with the aid of theoretical calculations based on Greens clear sky UV model. A simple formula was utilized to transform horizontal irradiances to vertical irradiances averaged over 360 azimuth angle. The model was verified with spectral and broadband measurements. The difference between the theoretical and measured UV radiation falling to horizontal surfaces was in most cases less than ±10%, and the additional error to theoretical vertical irradiances was less than ± 10%. The calculations show that the annual horizontal doses in Helsinki (60.2°N, 25°E) are about 35% higher than in Saariselkä (68.4°N, 27.5°E), but the difference is only 16% for vertical doses owing to the stronger contribution to vertical (facial) surfaces of the reflection of UV from snow. At Saariselkä, the maximum vertical irradiance at the end of April approaches the midsummer values. The ozone depletions up to 40% in February and March 1992 had no significant effect on the annual doses because the total ozone returned to normal before the UV increased to biologically significant levels.

INCREASED UV EXPOSURE IN FINLAND IN 1993

Abstract—

Intercomparison of lamp and detector‐based UV‐irradiance scales for solar UV radiometry

The high uncertainty of calibration is the most serious factor limiting accurate measurements of the solar UV, needed for the assessment of global UV trends. In this study, the calibration of FEL-type standard lamps traceable to three primary standard laboratories were compared with a transfer uncertainty of ±1.4% (2σ). In comparison with the reference lamp, the spectral UV irradiance scales agreed within 1.5%. However, the difference between a new lamp and the present reference was 2%, which is close to the limits of total uncertainty. It was interesting to observe that one of the scales, based on a cryogenic absolute radiometer, was in good agreement with the scales based on blackbody radiation sources. Examination of the long-term stability of the lamp-based UV scale in Finland showed a significant decrease of 6% in the scale when the standard lamp was changed to a lamp directly traceable to the primary standard of the National Institute of Standards and Technology (NIST). Previously, the lamp was traceable to the NIST through the secondary standard of Optronic Laboratories Inc. The main obstacle in improving the global conformity of UV scales is the instability of halogen standard lamps. On the basis of the present study, it is strongly recommended to use detector stabilization of the standard lamps and to investigate the use of portable detector standards in the intercomparisons between primary standards laboratories.

Detector‐Based Calibration Method for High‐Accuracy Solar UV Measurements

A novel method for calibrating the absolute responsivity of solar UV spectroradiometers has been developed and tested. The method is based on calibrated filter radiometers constructed from a detector, a precision aperture, a band‐pass filter and devices for temperature stabilization. The filter radiometers utilize a trap detector with very low reflectance. The filter and the detector can therefore be characterized separately. As an example the detector‐based calibration is compared at 312 nm wavelength with lamp‐based calibration by measuring the irradiance of a deuterium lamp with both the filter radiometer and the lamp‐calibrated spectroradiometer. The agreement between the results is at the level of 1%, well within the estimated uncertainties of both methods.

Portable detector‐based primary scale of spectral irradiance

A method developed at the Helsinki University of Technology (HUT) for calibrating standard lamps of spectral irradiance in UV-A and UV-B regions is presented. The method is based on a compact filter radiometer that is characterized absolutely to measure spectral irradiance. The filter radiometer can be brought to laboratories where accurate calibrations are needed. The method overcomes some of the instability problems encountered when using lamps as transfer standards, which makes it also useful in intercomparison campaigns. Test measurements are presented where two standard lamps issued by the National Institute of Standards and Technology (NIST), USA, are compared with the HUT scale held by the radiometer. The results suggest that the agreement between the two scales (HUT and NIST) is approximately 1%-2% in the wavelength region from 300 nm to 400 nm.

Performance tests of two Robertson-Berger-type UV meters Solar Light Model 500 and 501

Solar calibrations and laboratory tests were performed for erythemally weighted Robertson- Berger type UV radiometers. The test instruments included four Solar Light Model 500 and two Model 501 meters. The solar calibrations were performed at different elevation angles with relatively clear skies. The absolutely calibrated Optronic 742 spectroradiometer was used as a reference instrument and the measurement results were normalized to the temperature of 25 degree(s)C. The uncertainty of the spectroradiometric measurements is estimated to be +/- 8%. The absolute average calibration factors obtained varied from 1.05 to 1.16 for Model 500 meters and from 0.93 to 0.98 for two Model 501 meters. The spectral responsivity and cosine responses were found to be satisfactory for both meter types. To obtain better knowledge of the thermal properties of the Solar Light Model 500 meters a temperature sensor was installed at the underside of the green filter located just below the phosphor layer. The obtained temperature coefficient based on the green filter temperature measurements was 0.80%/ degree(s)C. The overall uncertainty of Solar Light Model 501 measurements is estimated to be +/- 11% and according to the phosphor temperature corrected SL 500 measurements +/- 14%. Without temperature correction the uncertainty of Model 500 measurements increases up to +/- 19%.

Effects of arctic ozone depletions in 1992 and 1993 on UV exposure in Finland

Exceptionally low total ozone up to 40% below the normal level has been measured over Northern Europe during winter and spring in 1992 and 1993. The increase in the UV exposure of the Finnish population associated with the combined effects of ozone depletion and snow reflection was examined in this study with the aid of broadband measurements and theoretical calculations. The theoretical calculations were verified with spectral and broadband measurements. The calculations show that the annual horizontal doses in Helsinki (60.2 degree(s)N, 25 degree(s)E) are about 30% higher than in Saariselka (68.4 degree(s)N, 27.5 degree(s)E), but the difference is only 12% for vertical doses owing to the stronger contribution to vertical (facial) surfaces of the reflection of UV from snow. In Saariselka, the maximum vertical irradiance at the end of April approaches the midsummer values. The ozone depletions had no significant effect on the biologically effective UV in 1992 since the total ozone returned to normal at the end of March before the UV increased to biologically significant level. In contrast, in 1993 low ozone levels were measured still at least up to mid May resulting in an average theoretical increase of 8% during a period from 14 April until 22 May in biologically effective UV.